Save render for compositing
Just to save your render and all your channels like mist, light passes, cryptomattes etc. to OpenEXR which allows to store all the data you need in order to comp your renders in a next stage or at any given time.
From Wikipedia:

OpenEXR is a high-dynamic range, multi-channel raster file format ... It is notable for supporting multiple channels of potentially different pixel sizes, including 32-bit unsigned integer, 32-bit and 16-bit floating point values, as well as various compression techniques which include lossless and lossy compression algorithms. It also has arbitrary channels and encodes multiple points of view such as left- and right-camera images.

Further reading:

OpenEXR in the Blender Manual
Save all render passes to EXR image?
What image format encodes the fastest, or at least faster? PNG is too slow